Injection molding machines and certain processes for injection molding are well known in the art. According to known injection molding methods, a molten material, such as plastic, is distributed to a mold having two halves which are then closed upon one another to form a part in the shape of the cavity that is formed in the mold.
More specifically, in known injection molding methods and processes, the molten material is supplied to the mold from an injection molding machine. The material is fed into the machine via a hopper or the like. The material is then transferred to a barrel associated with the machine where it is heated. The barrel houses a reciprocating screw which reciprocates linearly therein. During the injection molding process, the reciprocating screw moves toward the mold forcing the molten material through a manifold and then through a plurality of bushings into the mold cavity or cavities. The bushings are placed in the mold, at various locations depending upon the part to be molded.
It is necessary to control the amount of molten material that is supplied to the mold cavity, as the required amount depends on the size, shape, thickness and other factors of the part or parts to be molded. The amount of molten material that is supplied through each bushing is typically controlled by opening and closing each of the valve gates based on time control. The time at which the valve gate bushings open and close and the duration that they are open and closed is predetermined by the needs of the particular part to be molded and type of material being used. These are typically predetermined through experimentation, which takes a considerable amount of time.
For conventional injection molding methods utilizing a timer control, the opening period of the valve gates can only be controlled typically on the order of 0.01 seconds. Thus, due to various uncontrolled factors, such as changes in the flow resistance due to temperature and/or material viscosity changes, the amount of molten material flowing through the valve gates during a certain period of time cannot be accurately controlled. As a result, it is known that some rejects will be produced.
One known method is disclosed in U.S. Pat. No. 5,389,315. In this system, the valve gates of the bushings are sequentially opened one after the other based on the amount of molten resin supplied from the barrel as detected by metering mechanisms associated with each of the valve gates. Additionally, the patent discloses opening and closing the valve gates without any overlap based on the position of an in-line screw. As the valve gates are opened and closed without any overlap, the quality of the part can be affected, and the time to manufacture such a part can be increased. Further, the type of parts that can be made with such a process can be limited.
These prior methods are basically unable to consistently monitor and control the amount of material injected into the mold cavity with sufficient accuracy to produce a high quality molded part. Further, these methods and apparatuses require prolonged set up times and result in waste of material during the molding start-up.